Weekend Experiments
← All experiments
Medium3 and up90 min · Medium mess · indoorchemistryOct 25, 2026

Sugar glass

Boil sugar syrup until almost no water is left, pour it, and it cools into clear glass instead of crystals. Corn syrup keeps the sugar from lining up.

Sugar glass

The experience

About 20 minutes of boiling, then the pour, then the part worth waiting for: watching a clear sheet set in the pan. Once it is cool, it breaks into glass shards that go fast.

RUN THIS

How to run it

Boil sugar, water and corn syrup to hard crack, pour it thin onto a lightly oiled metal sheet pan, let it cool, and break it.

Setup

If you're out of corn syrup, use ¼ tsp cream of tartar or about ½ tsp lemon juice instead. Skip honey and agave, since they brown and stay sticky.

Steps

  1. 1

    Dissolve

    Put 2 cups sugar, 1 cup water and ½ cup light corn syrup in a heavy pot. Stir over medium heat until the sugar dissolves, then stop stirring.

  2. Saucepan on a stove with clear boiling sugar syrup and a candy thermometer clipped to the rim.
    2

    Boil

    Clip on the candy thermometer and boil to 300–310°F (hard crack). It takes about 20 minutes.

  3. Hot amber syrup pouring from a pot onto a lightly oiled metal sheet pan.
    3

    Pour

    Pour right away onto the lightly oiled metal sheet pan; it spreads to the edges, about 3–5 mm thick.

  4. Flat sheet of set amber sugar glass in a metal sheet pan on a countertop.
    4

    Cool

    Let it cool at room temperature, not in the fridge.

  5. Claw hammer on shattered amber sugar-glass shards in a metal sheet pan.
    5

    Break

    Once it is fully cool, break it. It goes tacky within a day in humid air.

Bear with me

Mechanism

Table sugar is sucrose. Left alone in a cooling syrup, sucrose molecules line up into crystals, and you get grainy candy. Corn syrup adds glucose and fructose, which are different shapes and get in the way. An acid like cream of tartar or lemon juice does the same job another way: it splits some of the sucrose into glucose and fructose right in the pot. Hard crack, 300–310°F, means very little water is left. When that syrup is poured thin and cools fast, the molecules have no time to line up. They freeze where they are, in a jumble, and the syrup becomes a glassy solid instead of crystals.

Name for this

Amorphous solid (a glass). Glucose and fructose act as crystallization inhibitors; the acid inverts sucrose.

Notes from home

Was fun. The cooking is 20 min, a bit long. But once it is poured and we watch it cool, it is fun. I made small candies and coated some banana in hard sugar with the leftovers. Probably more fun to add vanilla or something so it tastes less plain, or make actual caramel with the leftovers next time. He liked breaking it. It goes fast.